THERMAL BUCKLING OF SILICON CAPACITIVE PRESSURE SENSOR

The thermal behaviour of an absolute or differential capacitive pressu re sensor built from a micromachined silicon plate bonded to a Pyrex 7 740 substrate has been studied. Thermal drift due to the mismatch of t he thermal coefficients of expansion between the silicon, Pyrex substr ate and package has been analysed numerically with the finite-element method. Two types of boundary conditions at the Pyrex substrate-packag e interface (representing different die-mounting configurations) are s tudied: fixed and free base of the Pyrex substrate. The sensor with a fixed base presents a high temperature sensitivity beyond the buckling temperature, which is small for some dimensions (105 degrees C for a circular capacitive pressure sensor, having 1000 mu m diphragm radius, 12 mu m diaphragm thickness and 200 mu m Pyrex thickness). The influe nce of different dimensions of the sensor has been simulated to increa se the buckling temperature greatly. Buckling of the sensor with a fre e base occurs at negative temperatures and hence this sensor presents a feeble thermal drift at positive temperatures.